Fiber optic cables include optical fibers that transmit signals, for example, voice, video, and/or data information. Generally, it is desirable for fiber optic cables to have a high optical fiber count and be as compact as possible, thereby maximizing optical fiber density while still maintaining optical performance. Likewise, when routing fiber optic cables, it is generally desirable to have a compact configuration while maintaining optical performance. For example, in urban environments, fiber optic cables may be routed within a duct. A conventional duct design uses a larger duct that acts as a conduit for routing a number of inner ducts therein. Each inner duct may contain a single fiber optic cable or a plurality of such cables routed within a central passage of the inner duct.
Shown in FIG. 1 is a conventional 4-inch duct 10 having three conventional inner ducts 13 routed therein. Inner ducts 13, which are solid cylindrical tubes, have respective central passages 13a and are arranged generally in a triangular fashion within duct 10. Duct 10 is a solid cylindrical tube; however, duct 10 may be a passage through, for example, concrete. The size of ducts and inner ducts are generally given in inches, while cable outer diameters are generally given in millimeters and that nomenclature will generally be used herein.
The use of inner duct 13 prevents a fiber optic cable 12 from being tangled with other fiber optic cables when routed within duct 10. Moreover, inner ducts 13 are used within duct 10 because they give telecommunication providers flexibility in routing, replacing, and/or upgrading fiber optic cables that are routed within inner ducts 13.
Flexibility allows the telecommunication provider, for example, to replace an old generation fiber optic cable within inner duct 13 with a next generation fiber optic cable. In this case, the telecommunication provider can attach the next generation cable to an end of the old generation cable and remove the old generation cable from the other end of inner duct 13. While the old cable is removed from inner duct 13, the new cable is pulled into inner duct 13. Additionally, the telecommunication provider may leave one, or more, of inner ducts 13 empty, thereby being able to increase the optical fiber density within duct 10 when the need arises at a later date. Moreover, this advantageously allows the telecommunication provider to defer capital expenditures until increased capacity is required.
However, the use of conventional inner ducts 13 within duct 10 creates unused space within duct 10, thereby limiting the fiber optic density of duct 10. For example as shown in FIG. 1, the shaded area represents an unused space WS between inner ducts 13 and duct 10 that is not utilized for routing fiber optic cables. Moreover, the cross-sectional area of both the inner duct 13 and duct 10 is also unused space because it is not be utilized to route optical fibers.
An optical fiber density of inner duct 13 can be calculated by dividing the number of optical fibers within inner duct 13 by the area enclosed by an outer diameter of inner duct 13. Hence, the fiber optic density of a fiber optic cable routed within conventional inner duct 13 limits the fiber optic density of conventional inner duct 13. For example, an inner duct 13 having an outer diameter of 1.5 inches may include a fiber optic cable having 864 optical fibers with an outer diameter of about 27 mm (about 1.06 inches) or less routed therein. For the given example, the fiber optic density of inner duct 13 is about 0.758 optical fibers per square millimeter.
Likewise, an optical fiber density of duct 10 can be calculated by dividing the number of optical fibers within duct 10 by the area enclosed by an outer diameter of duct 10. By way of example, each inner duct 13 may have a central-tube fiber optic cable having 864 optical fibers routed therein, giving duct 10 a total of 2,592 optical fibers routed therein. In this example, 2,592 optical fibers within the convention 4-inch duct yield an optical fiber density of about 0.320 fibers per square millimeter.
When inner ducts 13 are full and further capacity is required it is expensive, and may be difficult, for the telecommunications provider to route another duct 10. Therefore, it is advantageous to have high optical fiber density within duct 10 and inner ducts 13.